1. Technical Field of the Invention
The present invention relates to very finely divided colorant compositions based on rare earth metal sulfides and comprising at least one alkali metal, to a process for the preparation thereof and to the use of same for the improved pigmentation of a wide variety of materials and substrates.
2. Description of the Prior Art
Inorganic colorant pigments are today widely used in many industries, especially in those of paints, plastics and ceramics. For these applications, such properties as, inter alia, thermal and/or chemical stability, dispersibility (ability of the material to disperse properly in a given medium), intrinsic color, coloring capacity and opacifying power constitute particularly significant criteria to be considered in the selection of a suitable pigment.
Unfortunately, the problem remains that the majority of the inorganic pigments which are suitable for applications such as those indicated above and which are currently effectively used on an industrial scale generally comprise metals (cadmium, lead, chromium and cobalt especially), the use of which is becoming increasingly strictly controlled, indeed banned, by law in many countries because of their allegedly very high toxicity. More particularly exemplary are the red pigments based on cadmium selenide and/or cadmium sulfoselenide and for which substitutes based on the rare earth metal sulfides are already known to this art. Compositions based on rare earth metal sesquisulfides and alkali metal elements have thus been described, in EP-A-545,746. These compositions, which are obtained via a process comprising heating a mixture based on a rare earth metal compound, an alkali metal element and sulfur, have proven to be particularly advantageous substitutes.
However, need continues to exist for substitute materials having pigment-grade properties which are yet further improved and which can be prepared via an industrially even simpler process.